As modern industry moves in the direction of developing large-scale structures, welding structures made of thick and ultra-thick plates have become increasingly widespread. Consequently, the bar of standard for welding methods has been raised higher, which is hard to be met by conventional welding methods. At present, large bevel multi-pass metal active gas (MAG) welding, metal inert gas (MIG) welding or submerged arc welding are usually employed to weld slab joints of pressure vessels, boilers, train rails, ships and other large thick-walled metal structural products. With the increasing plate thickness of welded structures, these methods have shown greater limitations and thus become inappropriate. This is most manifested by the fact that the dramatic increase of the bevel area of the thick plates has caused the needed welding work to increase by many folds, resulting in much consumed welding supplies, low productivity and high welding cost. In addition, various kinds of welding defects are consequently introduced, such as high heat input, coarse grains, wide heat-affected zone, poor mechanical properties of the welded joints, and cracks and so on.
Plasma arc welding is a high-energy beam welding method, and provides apparent improvements over conventional free arc welding in terms of energy density and directivity of the arc. In particular, keyhole plasma arc welding has shown eminent advantages in welding thick plates, thanks to its high energy density and excellent arc penetration. The generation of keyholes, a prominent feature of high-energy-density welding processes, has greatly increased the welding depth achievable in one-pass welding. However, as the plates grow thicker, a larger penetration depth is demanded, which nevertheless increases the tendency of keyhole instability. That is, a proper balance between the heat input and a stable generation of the keyholes is extremely difficult to achieve. When the work pieces to be welded grow to certain thickness, the plasma arc will no longer be capable of penetrating the work pieces. Therefore, there is a need to solve the problem of how to achieve stable keyhole generation of the plasma arc, so as to further increase the plate thickness that can be handled by a welding process using plasma arc.